In a 10 - 12 inch frying pan, combine onion, garlic and 1/4 cup broth. Stir often over medium-high heat until a brown film forms on pan bottom, 6 -8 mins. Deglaze pan by adding vinegar; stir to release brown film. Repeat browning and deglazing once or twice, using 2 T. broth at a time, until vegetables are richly browned.
Add beans to onion mixture and coarsely mash. If needed, add a little more broth to give beans the texture of creamy mashed potatoes. Add salt and pepper to taste. Mound in a bowl or on a plate. If making ahead, cover and chill up to 3 days.
Serve pate worm or cool;